An apparatus is known from U.S. Pat. No. 2,559,227 of RIEBER, for generating high frequency shockwaves, which apparatus comprises a truncated ellipsoidal reflector in which shockwaves are generated by discharge or electric arc between two electrodes converging to the first focal point of the ellipsoid, the object being to destroy a target situated in the second focal point of the ellipsoid, which is external to the truncated reflector (see FIG. 3 and col. 7, line 51 to col. 9, line 30).
Electrodes are produced in a highly conductive material such as copper or brass and are mounted on an insulator which is supported in pivotal manner by means of a device, so as to adjust the spacing between said electrodes (see col. 4, lines 42 to 53 and col. 8, lines 40 to 47).
With the RIEBER apparatus or any similar apparatus, the discharge or electric arc is produced between the electrodes and due to the sudden discharge of a capacitor, by closing a high voltage switch (see FIG. 2B). According to the RIEBER apparatus, the circuit between the electrodes comprises a capacitor, with an associated self-inductance. It has been noted that the capacitor discharge is of damped oscillatory type. In other words, the capacitor is going to discharge and to re-charge in reverse at a lower voltage than the initial voltage which is very high, until depletion of the charges contained in the capacitor.
Simultaneously, an electric arc and a plasma are established between the two electrodes of which the current will also be, by way of consequence, of damped oscillatory type, as can be understood with reference to FIGS. 1a, 1b and 1c. Accordingly, FIG. 1a illustrates the chronogram of voltages, while FIG. 1b illustrates the chronogram of currents established in the RIEBER type discharge circuit. It is found that when the circuit is closed at time t.sub.1, the voltage at the terminals of the electrodes rises suddenly to the value of the voltage at the terminals of the capacitors (see FIG. 1a). A low current is established between the two electrodes (FIG. 1b) due to the fact that, first the liquid in which the electrodes are immersed, and which is usually water, is still slightly electrically conductive, and second, that for reasons of safety and of arc ignition, a high resistance is provided in parallel to the capacitor supplying the electrodes.
After a certain time, namely after time t.sub.2, called latency time, the arc is established between the electrodes. At that moment, the current increases suddenly by several KA as clearly illustrated in FIG. 1b. It is a known fact that the arc is constituted by a plasma whose resistance is extremely low (about 1/100 or 1/1000 Ohm) and it is the low value of this resistance which explains the importance of the oscillations of current (FIG. 1b) and of voltage (FIG. 1a) during the discharge of a capacitor in an RL type circuit.
The energy contained and dissipated by the arc contributes to the vaporization of the liquid in which the electrodes are immersed, and which is normally water, to the creation of a steam bubble and consequently to the formation of the shockwave. The quicker this energy is dissipated, the more efficient will be the shockwave.
It is thus found that, due to the oscillatory nature of the current, as illustrated in FIG. 1b, the supply of energy to the external medium is progressive, as clearly illustrated in FIG. 1c.
This explains how, the quicker is the vaporization of the liquid, in particular water, the stronger will be the pressure wave and the shorter will be its pressure-rising time.
Thus, a great quantity of energy will have to be delivered to vaporize an important quantity of liquid, and in particular water.
Yet, virtually all the currently known devices use discharges which are all of damped oscillatory type, as illustrated in FIGS. 1a and 1b, resulting in a progressive dissipation of the energy with time (FIG. 1c).
In their prior document EP-A-0 296 912, the Applicants have proposed a first solution for delivering suddenly or in a relatively short time, most of the energy stored by the charge of the capacitor of the discharge circuit between two electrodes. It was proposed to this effect, to increase the electric resistance on the path of the electric arc at least between the electrodes by interposition of a high resistance insulating element, between the arc-generating electrodes. This solution is fully satisfactory when generating shockwaves whose initial pressure wave is substantially spherical.
However, said prior solution is difficult to implement mechanically because of the small dimensions of the electrodes and of the mechanical strength towards shockwaves. Moreover, the latency time problem is not solved in that the main aim of this particular solution is only to improve the discharge rate when electric arc is established, which does not improve the reproducibility of the discharge, hence of the shockwave.
Accordingly, the main object of the invention is to solve the new technical problem consisting in providing a solution permitting instant delivery in a relatively short time of most of the energy stored by the charge of the capacitor of the discharge circuit between two electrodes, by eliminating completely or substantially the latency time normally necessary for generating an electric discharge between the electrodes.
Another object of the invention is to solve the new technical problem consisting in providing a solution permitting complete or substantially complete elimination of the latency time when generating an electric discharge between two electrodes while considerably improving the reproducibility of the shockwave due to an important improvement in localizing the generation of the discharge current.
Yet another object of the present invention is to solve the new technical problem consisting in providing a solution permitting the complete or substantially complete elimination of the latency time when generating an electric discharge between the electrodes, while producing an electric discharge of critical damped type which will cause an instant delivery or a delivery in a relatively short time of most of the energy stored by the charge of the capacitor of the discharge circuit between the electrodes.
A further object of the present invention is to solve said new technical problems while providing a solution permitting a reduction of the wear of the electrodes, and limiting the extent of the alterations to be made on the existing prior apparatuses.
Yet another object of the invention is to solve the aforesaid new technical problems in an extremely simple manner which can be used on an industrial scale, particularly with reference to extracorporeal lithotripsy.
All said new technical problems have been solved for the first time by the present invention in a satisfactory manner, for little costs, and at industrial level, particularly with reference to extracorporeal lithotripsy.
Thus, in a first aspect, the present invention provides a method for improving the electric discharge rate produced in a liquid medium such as water, between at least two electrodes, generating such a discharge, characterized in that it consists in considerably reducing the resistance to the passage of the electric discharge at least between the electrodes in order to bring it to a resistance value near to the critical resistance by interposing at least between the electrodes, an electrically conductive liquid medium contained in an essentially closed reservoir surrounding the electrodes.
Said reservoir is produced in a material which will not substantially affect the propagation of the shockwaves. Examples of such materials are a latex, a silicon, or a metal strip, which are well known to skilled in the art.
According to another advantageous embodiment, the electrodes support the reservoir and are removable. They can therefore be supplied with the reservoir, the assembly then being usable and disposable, thus reducing maintenance costs compared with the prior solutions.
According to a particularly advantageous embodiment, the electrically conductive liquid medium used has an electrical resistance which is less than 1/10, and preferably at least 1/100 of the electrical resistance value of the ordinary ionized water used as reference. Preferably still, the electrical resistance of the electrically conductive medium according to the invention, as expressed in linear resistivity, is less than about 15 Ohm.cm. The electrically conductive liquid media can be constituted by an aqueous or non-aqueous electrolyte. A suitable aqueous electrolyte is water containing ionizable compounds, notably salts such as halide salts, for example NaCl, NH.sub.4 Cl, sulfates or nitrates with alkaline or alkaline earth metals or transition metals such as copper. A currently preferred electrically conductive aqueous liquid medium is constituted by water salted at the rate of 100 or 200 g/l, having respectively a linear resistivity value of 10 and 5 Ohm.cm.
More preference is given to an electrically conductive aqueous liquid medium containing about 10% by weight of NaCl (about 100 g/l) and between 0.5 and 2% by weight of phosphate salt, particularly disodium phosphate (Na.sub.2 HPO.sub.4, 12H.sub.2 O). The linear resistivity of such an electrically conductive medium is about 8 Ohm.cm. Advantageously, a dye, such as methylene blue, is added in the proportion of 2 mg/l in order to reveal any leaks in the reservoir.
Suitable non-aqueous conductive liquid media include the conductive oils, rendered conductive by the addition of conductive particles such as metallic particles, which are well known to those skilled in the art.
According to a second aspect, the present invention also provides a device for improving the rate of electrical discharge produced in a liquid medium such as water, between at least two electrodes generating such a discharge, characterized in that it comprises means for reducing the resistance to the passage of an electric discharge at least between the electrodes so as to bring it to a resistance value near to the critical resistance, comprising an essentially closed reservoir surrounding the electrodes, and filled with an electrically conductive medium. The material making up said reservoir is selected not to substantially affect the propagation of the shockwaves. In particular, said reservoir can be made of latex, silicon, or metallic strip. It can take the form of a membrane around the electrodes.
According to a third aspect, the present invention further relates to an apparatus generating shockwaves by electric discharge between at least two electrodes immersed in a liquid discharge medium, notably of extracorporeal type, characterized in that it comprises a device for improving the discharge rate as described previously. According to an advantageous embodiment, said apparatus comprises a truncated ellipsoidal reflector having an internal focal point where the shockwaves are generated by electric discharge between at least two electrodes and a focus, external to the reflector, in which the shockwaves are focussed, said truncated ellipsoidal reflector being filled with a liquid coupling medium. In this case, there is an essentially closed reservoir, as indicated hereinabove, which surrounds the electrodes and therefore the internal focus, which reservoir is filled with electrically conductive medium, while outside said reservoir, another liquid medium, notably water, is used inside the truncated ellipsoidal reflector.
Other characteristics of the electrically conductive medium according to the invention have been described with reference to the method and are obviously applicable to the device.
According to the invention, the discharge is produced through an electrically conductive medium, thus eliminating completely or substantially completely the latency time. Moreover, a considerable increase of the reproducibility of the shockwave generated between the electrodes is obtained. This is mainly due to the fact that in the conventional case, the arc is ignited at random in time and in space, inducing the formation of an inaccurately localized steam bubble, which is not the case according to the present invention. Also, according to the invention, the presence of an oscillating current is eliminated, so that the discharge is of critical damped type, as will be more readily understood from the description given with reference to the appended drawing.
Also according to the invention, the presence of the reservoir filled with electrically conductive liquid, enables the quantity of electrically conductive liquid used to be considerably reduced, and this liquid is not in contact with the patient. Moreover, the electric discharge takes place in a confined domain, thereby limiting electrical risks.
The invention therefore provides all the technical advantages indicated hereinabove, which were unexpected and non-obvious to the man skilled in the art.
Other aims, characteristics and advantages of the invention will also appear to the man skilled in the art from the following explanatory description made with reference to the accompanying drawings, particularly showing a presently preferred embodiment of the invention, given by way of example and non-restrictively